Description of the Conventional Techniques
Semiconductor lasers, solid lasers and gas lasers have been known as light sources outputting coherent light beams and are employed in various fields such as a measurement field, a communication field, and the like. Due to a restriction in the material used, the oscillation wavelengths generated by the above lasers are limited. In particular, it has been difficult to obtain light with short wavelength. Consequently, nowadays elements enabling the generation of a second-harmonic wave of the fundamental wave which are generated from the semiconductor laser and the like have been studied. The elements above hereinafter are referred to as a second-harmonic wave generating element or generator.
One example of such a second-harmonic wave generating element is disclosed in an article in Electronics Letters, Vol. 25, No. 3(Feb. 2, 1989) pp 174, 175. This article will be referred to as literature (a). The conventional second-harmonic wave generating element disclosed in literature (a) above has a region formed in a LiNbO.sub.3 substrate so as to function as a second-harmonic wave generator. The region has a nonlinear optical coefficient of a sign which is periodically reversed and, its period satisfies a quasi-phase-matching condition. The principle of a second-harmonic wave generation according to the quasi-phase-matching method had been theoretically foretold in an article by J. A. Armstrong et al. in Physical Review, Vol. 127, No. 6, pp. 1918-1932 (1962) prior to the literature (a) above. However, it was not until quite recently that the element was formed practically and a generation of the second-harmonic wave was confirmed as described typically in the literature (a) disclosing a second-harmonic wave generating element. Here, the construction of this second-harmonic wave generating element will be explained in detail with reference to FIG. 1 which is a perspective view depicting schematically the construction of the second-harmonic wave generating element.
The second-harmonic wave generating element uses a changing phenomenon of the sign of a nonlinear optical coefficient of LiNbO.sub.3 according to the direction of spontaneous ferroelectric polarization of the crystal. It is noted that the fact of a reversing of the spontaneous ferroelectric polarization of LiNbO.sub.3 crystal can occur when titanium (Ti) is diffused in a high temperature atmosphere (for example, about 1000.degree. C.) in the predetermined part of the LiNbO.sub.3 crystal has been disclosed in, for example, the literature "Japanese Journal of Applied Physics, Vol.6, No. 3 pp. 318-327 (1967)".
In the second-harmonic wave generating element disclosed in literature (a), the direction of the spontaneous ferroelectric polarization is reversed due to a titanium diffusion. Consequently, as shown in FIG. 1. on a +C surface of the LiNbO.sub.3 substrate 11 of a thickness 0.5 mm, titanium diffused regions 13 in a shape of stripes are ferroelectrically formed for example. The direction of spontaneous ferroelectric polarization in the striped regions 13, in which titanium is diffused, is reversed by the diffusion of titanium into the downward direction as shown in FIG. 1. On the contrary, the direction of spontaneous ferroelectric polarization in the regions 15, in which titanium is not diffused, remains in its original upward direction. As a result, spontaneous ferroelectric polarization periodic inverted (domain inverted) structures 13 and 15 are formed on the LiNbO.sub.3 substrate, resulting in a construction of a region 19 (hereinafter it may be referred to as a domain inverted region 19) in which the sign of a nonlinear optical coefficient is periodically reversed. The width of the regions 13 having titanium diffused and the width of the other regions 15 provided with no titanium dispersed or diffused are shown respectively by l.sub.n and l.sub.p. The coherence length of the fundamental waves is represented by l.sub.c. The quasi-phase-matching condition will be satisfied when the total periods l.sub.n and l.sub.p of the periodic structure 17 satisfy the following equation (1). EQU l.sub.n =l.sub.p =(2m+1)l.sub.c ( 1)
Consequently, it is preferable to form the periodic structure so as to make the number of periods of an odd number of the coherence length l.sub.c. The symbol m in the equation (1) above is zero or a positive integer. It is known that the coherence length l.sub.c is determined by the following equation (2) in, for example, the literature of Applied Physics Letters, Vol.47 (1985) pp 1125-1127. ##EQU1##
Wherein, .lambda. is a wavelength of the fundamental wave in a vacuum, n(2.omega.) is a refractive index of the LiNbO.sub.3 substrate relating to the second-harmonic wave, and n(.omega.) is a refractive index of the LiNbO.sub.3 substrate with reference to the fundamental wave.
Some examples of the concrete values of the coherence length l.sub.c are shown in the literature of Applied Physics Letters Vol.37 (1980) pp 607-609; when LiNbO.sub.3 is used and .lambda.=1.06 .mu.m, l.sub.c =3.4 .mu.m.
According to the literature (a) above, on the LiNbO.sub.3 substrate 11 of the second-harmonic wave generating element disclosed in the literature (a), an optical waveguide or wave-guiding route 21 (shown in FIG. 1 by a dashed-line) having a thickness substantially equal to that of the domain inverted region 19 is formed by a proton exchange method after the domain inverted region 19 is provided.
When a fundamental wave L.sub.1 is entered into a second-harmonic wave generating element according to the prior art through an end 11a, for example, shown in FIG. 1 as being perpendicular to the stripe direction of the striped region 13 of the domain inverted region 19, the fundamental wave L.sub.1 and the second-harmonic wave L.sub.2 of the fundamental wave emit through another end 11b on the emitting side of the element.
In addition, it was possible to enter a fundamental wave having of wavelength 1.06 .mu.m at 1 mW power through the optical waveguide 21, resulting in obtaining a second-harmonic wave with a wavelength of 532 nm of blue at 0.5 nW power.
However, according to the conventional second-harmonic wave generating element described and shown in FIG. 1, it is apparent that despite the incident fundamental wave of 1 mW power, the resulting second-harmonic wave has only 0.5 nW power as described above That is, the conversion efficiency (power of second-harmonic wave/power of incident fundamental wave) was merely or so low as 0.5.times.10.sup.-6 =0.5 nW/1 mW.
Further, considering coupling efficiency between the second-harmonic wave generating element and a light source (for example, laser means) supplying a fundamental wave to the element, the light power must emit a fundamental wave of a power higher than the power to be given to the emitting second-harmonic wave generating element. Consequently, the conversion efficiency between the power of the fundamental wave when emitted from the light source and another power of the second-harmonic wave emitted from the second-harmonic wave generating element becomes smaller than the conversion efficiency previously mentioned. As a result, the practical value of the second-harmonic wave generating element of the prior art decreases when it is used as a blue light emitting element.